Data compression is particularly important in the art of image storage and transmission. Images by their nature require or incur a large amount of data for their expression. A moderate resolution monotone image, for example, might consist of 640 picture elements, referred to as "pixels" or "PELs," per horizontal line. The monotone image typically includes 480 horizontal rows or lines, each containing 640 pixels per line. With 480 of such horizontal lines, a total of 307,200 pixels is displayed in a single 640.times.480 pixels image. If each pixel of the monotone picture requires one byte of data to describe it, a total of 307,200 bytes are required to describe just one black and white image.
Similarly, one standard color image of 640.times.480 pixels requires approximately 7.4 megabits of data to be stored or represented in a computer system. This number is arrived at by multiplying the horizontal and vertical resolution by the number of required bits to represent the full color range (or 640.times.480.times.24=7,372,800 bits). One frame of digitized NTSC (National Television Standards Committee) quality video comprises 720.times.480 pixels, which requires approximately one half megabyte of digital data to represent the image (720.times.480.times.1.5 bytes per pixel). In an NTSC system that operates at approximately 30 frames per second, therefore, digitized NTSC-quality video will generate approximately 15.552 megabytes of data per second.
Without compression, assuming a storage capability of one gigabyte and a two megabyte per second access rate, it is possible to either store 65 seconds of live video and to play it back at 3 frames per second, or store 21 high quality still images taking 24 seconds to store or retrieve each image. Lack of data compression in the transmission of such images forces the user to spend large amounts of time and money storing, sending and receiving the images. In practical terms this means that the user spends a lot of time waiting to receive each image. This is annoying to the user, and particularly disturbing if successive images need to be transmitted such as in the case of live or full-motion video transmission.
Many methods of compressing image data exist and are well known to those skilled in the art. Some of these methods are completely reversible, also known as "lossless" compression, in that they reverse upon decoding (decompressing) to exactly restore the original data without the loss or elimination of any data. These lossless compression techniques, however, cannot compress data to sufficiently large degrees without beginning to lose information. Because the reduction ratios are small, these lossless techniques cannot satisfy the transmission rates required for full-motion video transmission over analog telephone lines.
Other compression methods exist that are non-reversible, also known as "lossy" compression. These non-reversible methods offer considerable compression, but result in a loss of data due to their high rates of compression. The high compression rates are actually achieved by eliminating certain aspects of the image. There are generally two groups of non-reversible (i.e., lossy) compression techniques. One group applies transforms, such as the discrete cosine transform (DCT), to local areas of an image. Another group truncates or eliminates various of the resulting coefficients, thus reducing the amount data required for transmission. After transmission, an inverse transform is performed on the reduced data set to decompress and restore a reasonable facsimile of the original image. These lossy compression techniques can be combined with reversible methods for even greater levels of data compression. In general, however, the loss of data caused by the various prior art compression techniques is all too noticeable for transmitting a series of successive images, such as in the transmission of full-motion video. These methods are good at eliminating changes with a high spatial "frequency" but also generate substantial amounts of image data. The resultant compression ratios are good. However, the compression methods are very computation intensive, requiring significant processing power and/or much computation time.
Presently, commercially available modems allow a maximum of 33.6 Kbps (Kilobits per second) of data to be transmitted over a regular telephone ("POTS") line. Existing video compression systems employed for encoding and transmitting video over digital channels such as a T1 trunk or an Integrated Systems Digital Network (ISDN) line typically require much higher bandwidth (i.e., 56 Kbps or higher). A fixed bandwidth is typically allocated to video information. In a fiber distributed data interface (FDDI) with a bandwidth of 200 megabits per second, for example, 1.5 channels of live video can be accommodated or transmitted at the rate of one frame or image every two seconds. Conventional compression systems, therefore, cannot be used for encoding and transmitting video over ordinary analog telephone lines. To transmit full-motion video, one alternative is to use dedicated and special channels with existing video compression systems. The use of special and dedicated channels, however, is expensive.
Recent demands for full-motion video in applications such as video mail, video telephony, video teleconferencing, image database browsing, multimedia broadcasting, and other applications have required that improvements be developed for video compression so that video data can be successfully transmitted at appropriate transmission rates over a telephone line. It can be seen that data compression is still required in order to transmit and display full-motion video at 30 frames per second. Additional compression is required in order to reduce the amount of storage necessary, and increase the throughput, to transmit and display full-motion video in a quality closely approximating NTSC television. A need exists to achieve full-motion video transmission over analog telephone lines.
It is an object of this invention therefore to transfer full-motion video images over a two wire transmission medium such as a telephone line.
It is a further object of the invention to encode and compress real-time full-motion video using a camera or a television input from one computer station, through a telephone line, to another computer station, which decompresses and decodes the data to display full-motion video.